A uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol REUTERS/David MdzinarishviliA uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol, March 7, 2014. REUTERS/David MdzinarishviliA uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol, March 7, 2014. REUTERS/David MdzinarishviliA uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol, March 7, 2014. REUTERS/David MdzinarishviliA uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol, March 7, 2014. REUTERS/David MdzinarishviliA uniformed man, believed to be a Russian serviceman, stands guard near a Ukrainian military base outside the city of Sevastopol, March 7, 2014. REUTERS/David MdzinarishviliUniformed men believed to be Russian servicemen walk around a Ukrainian military base near Sevastopol, March 8, 2014. REUTERS/Baz Ratner (UKRAINE - Tags: POLITICS MILITARY)Uniformed men believed to be Russian servicemen stand guard at a Ukrainian military base near Sevastopol, March 8, 2014. REUTERS/Baz RatnerUkrainian servicemen walk on the roof of their base as a uniformed man believed to be a Russian serviceman stands guard at a Ukrainian military base near Sevastopol, March 8, 2014. REUTERS/Baz RatnerUkrainian servicemen walk on the roof of their base as uniformed men believed to be Russian servicemen (not seen in picture) stand guard at a Ukrainian military base near Sevastopol, March 8, 2014. REUTERS/Baz RatnerUkrainian serviceman look out of the gate of their military base near Sevastopol, March 8, 2014. REUTERS/Baz Ratner

Russian troops attempted to storm a Ukrainian missile base in Sevastopol on Friday night but were thwarted when the defenders barricaded themselves inside.

Stun grenades were said to have been thrown by the attackers who used a military truck to drive through the gates to the base but no shots were fired.

About 100 Ukrainian troops are thought to have been in the missile installation and the Russians left after holding talks with the base commanders and reaching an agreement with them. Estimates of the number of Russians involved ranged from 20 to 60.

It remained unclear why the Russians tried to enter the base, what their intentions were or what agreement they came to with the Ukrainian officers.

The Ukrainian deputy commander spoke briefly to journalists afterwards and said the Russians had told him they had permission to open fire.

Several members of a pro-Russian “self-defence” militia were also seen outside the base and were said to have attacked two press photographers who tried to take pictures of the scene.

Local women watch armed men, believed to be Russian soldiers, assemble near a Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas Peter (UKRAINE - Tags: POLITICS MILITARY TPX IMAGES OF THE DAY)Armed men, believed to be Russian soldiers, pose for a group photo as they assemble near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterA service man, believed to be a Russian soldier, lifts the canopy at the back of a military truck near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterArmed men, believed to be Russian soldiers, assemble near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterA uniformed man, believed to be a Russian serviceman, walks on guard outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014. REUTERS/Vasily FedosenkoUN special envoy Robert Serry gestures as he leaves in a car in Simferopol March 5, 2014. Serry was forced to abandon a mission to Ukraine's Russian-occupied Crimea region on Wednesday after being stopped by armed men and besieged inside a cafe by a hostile crowd shouting "Russia! Russia!" REUTERS/Vasily FedosenkoUN special envoy Robert Serry gets in a car in Simferopol March 5, 2014. Serry was forced to abandon a mission to Ukraine's Russian-occupied Crimea region on Wednesday after being stopped by armed men and besieged inside a cafe by a hostile crowd shouting "Russia! Russia!" REUTERS/Vasily FedosenkoUN special envoy Robert Serry gestures as he leaves in a car in Simferopol March 5, 2014. Serry was forced to abandon a mission to Ukraine's Russian-occupied Crimea region on Wednesday after being stopped by armed men and besieged inside a cafe by a hostile crowd shouting "Russia! Russia!" REUTERS/Vasily FedosenkoArmed men, believed to be Russian soldiers, pose for a group photo as they assemble near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterArmed men, believed to be Russian soldiers, assemble near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterArmed men, believed to be Russian soldiers, assemble near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterArmed men, believed to be Russian soldiers, assemble near an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterAn armed man, believed to be a Russian soldier, stands guard outside an Ukrainian military base in Perevalnoe March 5, 2014. REUTERS/Thomas PeterUniformed men, believed to be Russian servicemen, march outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014.Uniformed men, believed to be Russian servicemen, march outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014. REUTERS/Vasily FedosenkoUniformed men, believed to be Russian servicemen, march outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014. REUTERS/Vasily FedosenkoUniformed men, believed to be Russian servicemen, march outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014. REUTERS/Vasily FedosenkoUniformed men, believed to be Russian servicemen, march outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014. REUTERS/Vasily FedosenkoUniformed men, believed to be Russian servicemen, smoke outside a Ukrainian military base in the village of Perevalnoye outside Simferopol, March 5, 2014. REUTERS/Vasily Fedosenko

The standoff came at the end of a day when Russia appeared to have tightened its grip on the Crimean peninsula with an estimated 20,000 of its troops now based in the region. The estimate by the Pentagon was 10,000 lower than that of a Ukrainian figure cited earlier today.

Moscow’s strained relations with Western nations continued to fester as it issued warnings that sanctions or other measures imposed by the United States or the European Union would provoke retaliation.